DARAA, Syria - A roadside bomb hit a Syrian military truck Wednesday just seconds after the head of the U.N. observer team drove by in a convoy, demonstrating the fragility of the international plan to end the country's bloodshed.

In Washington, President Obama took steps to extend sanctions against the government of President Bashar al-Assad, saying Syria posed an "unusual and extraordinary threat" to U.S. national security and diplomatic goals.

The attack, which the regime said wounded 10 Syrian soldiers, emphasized the limits of the international community's plan to use unarmed observers to promote a cease-fire between government troops and rebels trying to topple Assad.

The team of 70 U.N. military observers in Syria should grow to more than 100 in the coming days. It is unclear when the full team of 300 will arrive. They are to oversee a U.N.-brokered cease-fire that was intended to allow for talks on a political solution to the conflict but began unraveling shortly after it was due to take effect April 12.

U.N. Secretary-General Ban Ki-moon condemned the attack, saying the continuing violence undermined the plan, which is "possibly the only remaining chance to stabilize the country and avert a civil war."

The peace plan has been troubled from the start, with government troops shelling opposition areas and rebels attacking military convoys and checkpoints despite the cease-fire.

Many civilians have grown critical of the plan, saying it does not protect them.

U.S. State Department spokeswoman Victoria Nuland said the roadside blast that hit troops escorting the U.N. monitors was "further evidence that the cease-fire is not holding."

Wednesday's blast, witnessed by an Associated Press reporter traveling with the United Nations, gave a close-up example of the attacks on security forces that are almost daily events.

The bomb went off as Maj. Gen. Robert Mood, the Norwegian head of the U.N. observer team, rode in a clearly marked armored white Land Cruiser from the capital, Damascus, to the southern city of Daraa, where Syria's uprising began.

Speaking to reporters later, Mood said it was unclear whom the bomb was targeting. "For me the important thing is really not speculating about who was the target, what was the target, but it is to make the point that this is what the Syrian people [are] seeing every day, and it needs to stop," he said.